You couldn't credibly use statistics to discount guns that do in fact shoot at or better than 1/2moa to XXX distance. There are plenty of shooting systems that perform this well.
For those guns that only occasionally shoot at that level, again, it doesn't take statistics to see that. It doesn't take 20 shots, or 100, or 1,000 to know it, better or worse.
I've read it suggested that you have to shoot at least 20shot groups to see what a system is capable of. Well a lot happens in 20 shots that may not with 1 or 3 or 5 shots at a time. So suggested is really a different standard,, such as: 20 shot 1/2moa to XXX distance.
Then when someone implies their gun shoots 3 shot 1/2moa at 300yds 'when I do my part', they just might be correct. Hell they may be truthful with a boast of 5 shot 1/4moa at 100yds, with their worst effort.
Just prior to a local hunting accuracy contest, some older than dirt BR shooter drops down a 6PPC(made of dirt I think) on bags and fired 5 shots touching at 200yds -while not even behind the gun with each shot.. He stood to the side, loading single shot, cycling the bolt, touching the trigger, 5 times. I gave him light hearted crap about missing the bullseye, and thinking to myself; 'No chance against him'.
But on the 9th round of cold bore single shot accuracy (into a 3/4 bull at 200), he knocked himself out of the competition. Threw a shot somehow, or maybe he had a little too much to drink by then, or maybe he was just being nice. No tellin really.
He was lending the gun to 2 other hunters, and I finally beat the last one of them at round 14(getting dark by then). This was easier, because they were not shooting free recoil, and they didn't have my better scope.
I don't shoot that contest anymore, as it's now dominated by 30BRs instead of hunting guns.
My point in all this: I didn't beat the gun. I beat the shooters.
And anyone with that gun could actually brag that it would shoot 1/2moa if they did their part,, all day long.
They're out there, it's just not all that common